1. Technical Field
The present invention relates to data coding. More particularly, the present invention relates to subband filtering during data encoding and decoding.
2. Description of the Prior Art
Even though modern storage media allow vast quantities of information to be stored in a minimal amount of physical space, it is still desirable to use coding techniques to effect data compression. In this way stored materials may be presented in a variety of convenient standard formats.
Decoding such coded materials requires that the stored materials be reconstructed by application of various decompression techniques. Such reconstruction is compute intensive, must be reliable, and must take place fast enough, for example in the case of audio and video information, to allow real time presentation of the stored materials. Additionally, the decode process must not require excessive or expensive hardware to meet the requirements of speed and accuracy.
The coding format chosen can make considerable difference in meeting the twin goals of speed and accuracy. The format used to code digital audio and video signals for storage and retrieval is dictated by various standards. For example, the Motion Picture Expert Group (MPEG) has established a standard for audio and video coding. The MPEG audio coding standard provides high quality audio using reduced bit rates of 128 kbit/s per audio channel instead of the 706 kbit/s required in uncompressed Compact Disc.RTM. quality audio. See ISO/IEC 11172-3, "Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbit/s--Part 3: Audio, "ISO/IEC JTC 1/SC29, May 20, 1993; and H. Musmann, The ISO Audio Coding Standard, IEEE GLOBECOM Conference, pp. 511-517, San Diego, Calif. 1990.
In the MPEG audio coding standard, a digital audio signal is encoded to produce a compressed bitstream for storage on any desired medium, e.g. compact disk, tape, etc. During MPEG audio encoding a nonstandard encoding technique may be used for such tasks as estimation of the auditory masking threshold, quantization, and/or scaling. The MPEG standard requires that the encoded signal must be such that a decoder conforming the specifications of the standard can produce audio suitable for the intended application from such encoded signal.
While significant effort has been spent in improving the video decoding and display techniques for MPEG files, very little effort has gone into improving the speed and efficiency of MPEG audio coding. As a result, audio decoding initially represented more than 40% of the overall decoding time. Various improvements have been made to MPEG audio decoding, such that decoding the audio portion of an MPEG coded video sequence amounts to 33% of the overall decoding time; while video coding amounts to 55% of the overall decoding time, and synchronization amounts to 12% of the overall decoding time. Fast execution of the MPEG standard is very important for real-time or close to real-time encoding and decoding in most applications where the use of special hardware would make the application of such technology too expensive.
Subband filtering is one of the most compute intensive operations in the MPEG audio coding standard and therefore an area of critical concern in improving coding performance for audio materials. One especially compute intensive operation of subband filtering is the matrixing operation. Matrixing is presently accomplished by brute force computation. That is, in the MPEG standard for audio 32.times.64=2,048 multiply-accumulate operations must be performed. Performing these calculations while still meeting the requirements of speed and accuracy adds significant complexity and expense to the design of the hardware that implements these functions.